Zions Bank "aided and abetted" the scheme and "breached" its duty to take action once it became aware Rust diverted the funds to himself, family members and "flailing" companies, the lawsuit says.

"Rust could not have perpetrated the silver pool scam on his own. Instead, he crucially depended on the knowing participation of his bank, Zions Bank, through which Rust committed his fraud," according to the lawsuit.

Rosca said the bank might have let the activity continue because there's a lot of money to be made from a customer with an account the size of Rust's.

"That's a customer that the bank would want to keep happy," he said.

Rosca said the investors sued the bank because they have no reason to believe Rust Rare Coin has any assets and a court order in the civil case bars litigation against the company while it is in receivership.

"In reality, however, plaintiffs’ investment in the silver pool was a sham," the lawsuit says. "Rust was not buying or selling silver, nor did he have physical silver stored in a Brink’s warehouse."